One of the trends in modern wireless communication systems is frequency band extension with simultaneous carrier frequency shift to the millimeter-wave range. In the millimeter-wave region (30-300 GHz) of the electromagnetic spectrum, such applications as indoor local radio networks, radio relay links, automotive radars, microwave imaging devices etc. are already successfully used. For example, communication systems operating in the millimeter-wave range provide significant improvement in data transmission throughput of up to several and even tens of Gb/sec.
Millimeter-wave communication systems and radars have recently found widespread use due to developments in semiconductor technology and the possibility of Transmitter/Receiver (Tx/Rx) implementation on semiconductor integrated circuits (IC) instead of traditional waveguide components of discrete functional parts. Such ICs are usually mounted on dielectric boards, thus forming fully integrated devices. The interconnection between ICs on a dielectric board in most cases is realized by microstrip transmission lines. Meanwhile, some elements of radio devices (for instance, antennas) should principally comprise waveguide interfaces to provide required characteristics (for example, high gain, low loss or high radiated power in case of antennas).
Thus, in order to provide efficient function, millimeter-wave communication systems require an effective waveguide-to-microstrip transition for electromagnetic signal transfer in any direction between a waveguide and a planar transmission line realized on a dielectric board. Moreover, in addition to radio communication systems and radars, such transitions are used in microwave measurement equipment where waveguides are utilized as low-loss transmission lines.
General requirements for waveguide-to-microstrip transitions used in modern millimeter-wave communication systems include wide operational bandwidth, low level of insertion loss, low fabrication cost in mass production and simple construction for easy integration into the communication device.
Some configurations of known waveguide-to-microstrip transitions which can be used in millimeter-wave devices are considered below.
A waveguide-to-microstrip transition based on a stepped waveguide structure (so-called “ridged waveguide”) is known from the paper “A Novel Waveguide-to-Microstrip Transition for Millimeter-Wave Module Applications” written by Villegas, F. J., Stones, D. I., Hung, H. A. published in IEEE Transactions on Microwave Theory and Techniques, Vol.: 47, Issue 1, January 1999. A dielectric board with a microstrip line is positioned along the waveguide longitudinal axis. The line is electrically connected to the highest step of the ridged waveguide. Drawbacks of such transition include high complexity and therefore high manufacturing cost. Furthermore, there are some issues related to the positioning of the board in the waveguide channel leading to worse performance and poor repeatability. These disadvantages are further amplified with the increase of operational frequencies to the millimeter-wave range.
Another waveguide-to-microstrip transition (“Design of Wideband Waveguide to Microstrip Transition for 60 GHz Frequency Band” written by Artemenko A., Maltsev A., Maslennikov R., Sevastyanov A., Ssorin V., published in proc. of 41st European Microwave Conference, 10-13 Oct. 2011) is based on a planar radiating element placed inside an aperture of a waveguide channel. The electromagnetic coupling between the radiating element and the microstrip line is provided by a slot cut in the metal ground layer of the microstrip line. The transition is relatively narrowband due to the resonance nature of the slot and the radiating element. Moreover, such a transition requires several dielectric layers on the board, thus increasing structure complexity and sensitivity of the transition to manufacturing error. Finally, the presence of the dielectric board inside the waveguide channel leads to additional signal loss related to dielectric loss in the substrate.
Yet another waveguide-to-microstrip transition is known from the paper “Wideband Tapered Antipodal Fin-Line Waveguide-to-Microstrip Transition for E-band Applications” written by Mozharovskiy A., Artemenko A., Ssorin V., Maslennikov R., Sevastyanov A., published in proc. of 43rd European Microwave Conference, 6-10 Oct. 2013. In this transition, a dielectric board with a printed microstrip line is clamped between two metal parts forming a waveguide channel along the transmission line. Due to such an arrangement, the transition experiences a high level of parasitic radiation from the board end face that leads to significant insertion loss. Moreover, the need for manufacturing two metal parts forming a waveguide channel leads to strict requirements for flatness and surface roughness which lead to an increase in manufacturing costs.
The closest prior-art of the present invention is a waveguide-to-microstrip transition described in the U.S. Pat. No. 6,967,542 filed on Dec. 30, 2004. The prior-art transition is composed of a dielectric board with a microstrip line and a microstrip probe which is placed between an input waveguide and a short-circuited waveguide of similar cross-section profile. The shorted waveguide is located at the same board side with the line and the probe. At the same time, the input waveguide which is often formed by the interface of a specific bulky radio communications device is arranged on the microstrip ground side of the board. Such mutual arrangement of the transition elements provides enough space on the board for IC integration, with such ICs connectable to the microstrip line. The input waveguide piece can comprise a flange arranged on the dielectric board and providing electrical contact between the waveguide and the microstrip ground directly or via through-holes made in the board.
The main drawback of the transition described in the U.S. Pat. No. 6,967,542 filed on Dec. 30, 2004 is the emergence of an equivalent LC circuit (resonant circuit) formed by the waveguides and a portion of the dielectric board that is located inside the waveguide channel. The resonant nature of the LC circuit limits the operational bandwidth of the device and therefore necessitates the use of additional features on the board providing an extension of the transition operational bandwidth. For example, in the prior-art transition, a microstrip quarter-wave impedance transformer, different matching microstrip stubs etc. are utilized for this purpose. These elements significantly complicate the transition design and decrease manufacturing tolerances. Another disadvantage is an increase in insertion loss between the line and the waveguide which is caused by the presence of the dielectric board substrate in the waveguide channel area.
Thus, there is a need for a probe-type waveguide-to-microstrip line transition providing a wide operational bandwidth and low insertion loss with a structure that does not contain any parasitic capacitance of the impedance between the probe and the waveguide channel. In such a transition, there is no need for special parasitic capacitance compensation techniques, thus significantly simplifying device structure, easing the precision requirements in manufacturing and mutual positioning of the board with the microstrip line with respect to the waveguide channel.